Capacitor

ABSTRACT

In a capacitor comprising a pair of electrodes and a dielectric substance intervening between the two electrodes, one of the electrodes is composed of sintered niobium nitride. Preferably, the dielectric substance is composed of niobium oxide and the electrode other than the electrode composed of sintered niobium nitride is composed of an ingredient selected from electrolytes, organic semiconductors and inorganic semiconductors. This capacitor has good environmental stability and good leak current characteristics.

This is a divisional of application Ser. No. 10/012,285, filed Dec. 12,2001, now U.S. Pat. No. 6,452,777 which is a continuation of U.S.application Ser. No. 09/775,493 filed Feb. 5, 2001, now U.S. Pat. No.6,347,032, which is a continuation of U.S. application Ser. No.09/620,898 filed Jul. 20, 2000, now abandoned, which is a divisional ofU.S. application Ser. No. 09/171,902 filed Oct. 28, 1998, now U.S. Pat.No. 6,115,235, which is a National Stage Application filed under §371 ofPCT Application No. PCT/JP98/00823 filed Feb. 27, 1998, the disclosuresof all of which are incorporated herein by reference.

TECHNICAL FIELD

This invention relates to a novel capacitor. More particularly, itrelates to a capacitor which is inexpensive and exhibits good leakcurrent characteristics, and a capacitor which has a large capacitance,especially a large capacitance per unit weight at a high frequency, andexhibits good leak characteristics.

BACKGROUND ART

As an electrode of a capacitor made of a sintered metal, those which arecomposed of sintered aluminum, tantalum and alloys thereof are known.These capacitors have widely used in various fields. For example, for acapacitor used in a smoothing circuit for obtaining a direct currentfrom an alternating current, it is desired that the capacitor possessesa low impedance and a large capacitance at a high frequency forsuppressing the occurrence of spike-shaped voltage and enhancing theefficiency of conversion to a direct current.

The above-mentioned sintered metals used as a capacitor electrode haveproblems. Namely, sintered aluminum has poor environmentalcharacteristics such as moisture resistance and chemicalcharacteristics, and sintered tantalum is expensive. Sintered niobium isalso known as a material used for a capacitor electrode, and notpossessing the problems encountered with sintered aluminum and tantalum,but, it has another problem that oxygen adsorbed on its surfaceinfluences dielectrics as mentioned below, and thus, the leak currentcharacteristics are not satisfactory and it is of poor practical use.

To provide a capacitor used in a smoothing circuit and having anenhanced capacitance at a high frequency, the volume of a sintered metalsubstrate made of, for example, tantalum or aluminum, should beincreased. The increase in volume of the sintered metal substrate isinconsistent with a requirement of miniaturization of a capacitor. Amongothers, tantalum gives a relatively satisfactory for the requirements ofan enhanced capacitance at a high frequency and a miniaturization of acapacitor, but, it is still not completely satisfactory for theserequirements. Usually a tantalum oxide is used as a dielectric substancefor a capacitor with an electrode composed of sintered tantalum bodies.However, if a material having a dielectric constant larger than that oftantalum oxide is used as a dielectric substance, the capacitor can bemore miniaturized. As examples of the material having a large dielectricconstant, there can be mentioned titanium oxide and niobium oxide. But,these materials exhibit poor leak current (hereinafter abbreviated to“LC”) characteristics.

DISCLOSURE OF INVENTION

The inventors have found, first, that sintered bodies of niobium nitrideare advantageous in that the amount of oxygen deposited on the surfacethereof is minor and the leak current characteristics of the capacitorare satisfactory, and secondly, that the above-mentioned problem as forLC characteristics of a capacitor with niobium oxide dielectrics is dueto the fact that oxygen deposited on the surface of sintered bodiesinfluences the dielectric substance. Based on these findings, theinventors have completed the present invention.

The inventors have further found that, if the electrode other than theelectrode composed of sintered niobium nitride bodies is made of atleast one compound selected from organic semiconductors and inorganicsemiconductors, which do not have a capability of supplying oxygen to anundue extent, a capacitor having a large capacitance at a high frequencycan be obtained. Further, if, as the organic semiconductor or theinorganic semiconductor, those which have an electrical conductivity of10⁻² S·cm⁻¹ to 10³ S·cm⁻¹ are used, a capacitor having a more reducedimpedance can be obtained.

Thus, in accordance with the present invention, there is provided acapacitor comprising a pair of electrodes and a dielectric substanceintervening between the electrodes, characterized in that one of theelectrodes is composed of sintered niobium nitride.

The dielectric substance of the above-mentioned capacitor is preferablymade of niobium oxide, more preferably made of niobium oxide prepared byelectrolytic oxidation of the sintered niobium nitride. The other of thetwo electrodes is preferably made of at least one ingredient selectedfrom electrolytes, organic semiconductors and inorganic semiconductors,more preferably at least one ingredient selected from organicsemiconductors and inorganic conductors, which have an electricalconductivity of from 10⁻² S·cm⁻¹ to 10³ S·cm⁻¹.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a partially cutaway view in perspective specificallyillustrating one example of the capacitor of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The niobium nitride constituting one of the electrodes in the capacitorof the invention is made by partially nitrifying metallic niobium. Forexample, niobium nitride of a powdery form is made by nitrifying thesurfaces of particles of powdery niobium in a nitrogen gas atmosphere.In this instance, the amount of nitrogen bound to the niobium is in therange of from 10 to 200,000 ppm by weight, preferably 100 to 50,000 ppmby weight. For nitrifying niobium into niobium nitride having thedesired nitrogen content, the temperature employed is not higher than2,000° C. and the time employed is in several tens of hours. Generally,as the temperature for nitrification becomes high, the particle surfacesof powdery niobium are nitrified in a shorter time. Even at roomtemperature, when powdery niobium is fluidized for several tens ofhours, powdery niobium nitride containing several hundreds ppm ofnitrogen is obtained.

The thus-obtained powdery niobium nitride has a shape approximatelysimilar to that of the powdery niobium used as the raw material. In oneexample, if a powdery niobium mass obtained by pulverizing a niobiumlump is used as a raw material, powdery niobium nitride having variousshapes which are peculiar to pulverized mass is obtained. In anotherexample, if powdery niobium in the form of a secondary particle is used,which is prepared by reducing potassium fluoroniobate to give a finelydivided particles, and granulating the finely divided particles intosecondary particles, then, powdery niobium nitride similar to thesecondary particles is obtained. Further, for example, if powderyniobium having an average particle diameter of from 0.5 μm to 100 μm isused, powdery niobium nitride having a similar average particle diameteris obtained.

The sintered niobium nitride is obtained by sintering, for example,powdery niobium nitride at a high temperature in vacuo. In one example,powdery niobium nitride is press-molded and then the molded product isallowed to stand at a temperature of 1,000 to 2,000° C. and a pressureof 10⁻¹ to 10⁻⁶ Torr for several minutes to several hours to give asintered niobium nitride. If the degree of vacuum is insufficient atsintering, air is entrapped in the powdery material during sintering,oxidation occurs simultaneously with nitrification with the result thatthe capacitor with the niobium nitride electrode has a poor performance.Generally a suitable sintering temperature varies depending upon theparticle diameter of the powdery niobium nitride, and, the smaller theparticle diameter, the lower the sintering temperature.

As the dielectric substance used in the capacitor of the invention,there can be mentioned, for example, tantalum oxide, niobium oxide,polymeric substances and ceramic compounds. When tantalum oxide is usedas a dielectric substance, the tantalum oxide can be prepared bydepositing a tantalum-containing complex such as, for example, an alkoxycomplex or an acetylacetonato complex on an electrode, and then,subjecting the deposit to hydrolysis and/or pyrolysis. When niobiumoxide is used as a dielectric substance, the niobium oxide can beprepared by chemically converting a niobium nitride electrode intoniobium oxide in an electrolyte, or by depositing a niobium-containingcomplex such as, for example, an alkoxy complex or an acetylacetonatocomplex on an electrode, and then, subjecting the deposit to hydrolysisand/or pyrolysis. Thus, a niobium oxide dielectrics can be formed on thesurface of niobium nitride electrode by converting a niobium nitrideelectrode into niobium oxide in an electrolyte or subjecting aniobium-containing complex on a niobium nitride electrode to hydrolysisand/or pyrolysis. The conversion of niobium nitride into niobium oxidein an electrolyte can be effected usually by using an aqueous protonicacid, for example, an aqueous 0.1% phosphoric acid solution or sulfuricacid solution. In the case where niobium nitride is formed into theniobium dielectrics in an electrolyte, the capacitor of the invention isan electrolytic capacitor with a positive electrode composed of niobiumnitride. In the case where a niobium-containing complex is subjected tohydrolysis and/or pyrolysis to yield niobium oxide, the niobium nitridehas theoretically no polarity and can be used either as a positiveelectrode or a negative electrode.

The polymeric substance dielectrics can be prepared by, as described inJapanese Unexamined Patent Publication No. H7-63045, a process wherein agaseous or liquid monomer is introduced in voids or pores within metal,followed by polymerization; a process wherein a solution of a polymericsubstance in a suitable solvent is introduced; and a process wherein amolten polymeric substance is introduced. As specific examples of thehigh polymeric substances, there can be mentioned a fluororesin, analkyd resin, an acrylic resin, a polyester resin such as polyethyleneterephthalate, a vinyl resin, a xylylene resin and a phenol resin.

The dielectric substance composed of a ceramic compound can be preparedby a process for producing a compound with perovskite structure on asurface of metal having voids or pores, as described in JapaneseUnexamined Patent Publication No. H7-85461. As specific examples of thecompound with peroviskite structure, there can be mentioned BaTiO₃,SrTiO₃, MgTiO₃ and BaSnO₃.

The electrode other than the niobium nitride electrode of the capacitorof the invention is not particularly limited, and can be composed of atleast one ingredient selected from electrolytes well known in analuminum electrolytic capacitor industry, organic semiconductors andinorganic semiconductors. As specific examples of the electrolytes,there can be mentioned a mixed dimethylformamide/ethylene glycol liquidcontaining 5% by weight of isobutyltripropylammonium borontetrafluoride,and a mixed propylene carbonate/ethylene glycol liquid containing 7% byweight of tetraethylammonium borontetrafluoride. As examples of theorganic semiconductors, there can be mentioned an organic semiconductorcomposed of benzopyroline tetramer and chloranil, an organicsemiconductor predominantly comprised of tetrathiotetracene, an organicsemiconductor predominantly comprised of tetracyano-quinodimethane, andorganic semiconductors predominantly comprised of electricallyconductive polymers represented by the following formula (1) or (2),which are doped with a dopant.

wherein R¹, R², R³ and R⁴ independently represents hydrogen, an alkylgroup having 1 to 6 carbon atoms or an alkoxy group having 1 to 6 carbonatoms, X represents an oxygen, sulfur or nitrogen atom, R⁵ representsonly when X is a nitrogen atom, and represents hydrogen or an alkylgroup having 1 to 6 carbon atoms, R¹ and R² may form together a ring,and R³ and R⁴ also may form together a ring. As specific examples of theelectrically conductive polymers of formulae (1) and (2), there can bementioned polyaniline, polyoxyphenylene, polyphenylene sulfide,polythiophene, polyfuran, polypyrrole and polymethylpyrrole. As examplesof the inorganic semiconductors, there can be mentioned inorganicsemiconductors predominantly comprised of lead dioxide or manganesedioxide, and inorganic semiconductors composed of triiron tetraoxide.These semiconductors may be used either alone or as a mixture of atleast two thereof.

When organic semiconductors and inorganic semiconductors having anelectrical conductivity of 10⁻² S·cm⁻¹ to 10³ S·cm⁻¹ are used as theorganic semiconductors and inorganic conductors, capacitors having a farreduced impedance and a more enhanced capacitance at a high frequencyare obtained.

The structure of the capacitor of the invention may be those which haveheretofore been employed, provided that the capacitor comprises a pairof electrodes and a dielectric intervening between the electrodes. Onespecific example of the capacitor of the invention is illustrated inFIG. 1, wherein a sintered niobium nitride 1 composed of a plurality ofsintered niobium nitride bodies is placed as an electrode and on whichniobium oxide dielectric layers have been formed by chemicallyconverting the surfaces of the sintered niobium nitride bodies intoniobium oxide in an electrolyte, or by subjecting a niobium-containingcomplex to hydrolysis and/or pyrolysis to produce niobium oxide on thesurfaces of the sintered niobium nitride bodies. The other electrode isformed on the dielectric layer.

Further, a carbon paste 2 and a silver paste 3 are formed in this orderon the other electrode, and then, the thus-prepared laminated product isencapsulated with a sealing material such as epoxy resin to form acapacitor. The capacitor is provided with a niobium lead 4 which hasbeen sintered in integrated with the sintered niobium nitride bodies orwhich has been welded to the niobium nitride sintered bodies. Thecapacitor is assembled together with a positive electrode lead 5 and anegative electrode lead 6 and the assembly is enclosed by an outer resincovering 7.

The capacitor provided with the niobium lead 4, illustrated in FIG. 1,is a rectangular parallelopiped, but, its shape is not particularlylimited thereto and may be, for example, cylindrical.

The capacitor of the invention will now be described specifically by thefollowing examples.

EXAMPLES 1 TO 7

Powdery niobium having an average particle diameter of 10 to 40 μm wastreated at 400° C. in a nitrogen atmosphere to give powdery niobiumnitride. The amount of nitrogen bound to niobium by nitrification wasabout 2,000 ppm by weight. The powdery niobium nitride was sintered at1,500° C. in vacuo to give sintered niobium nitride bodies having adiameter of 10 mm and a thickness of about 1 mm, and containing poreshaving an average diameter of 3 μm with a porosity of 45%. The sinteredniobium nitride bodies were treated in an aqueous phosphoric acidsolution at a voltage of 20 V to form a niobium oxide dielectric layeron the surface of each sintered body.

Each of the substances for forming an electrode other than the electrodecomposed of the sintered niobium nitride bodies, as shown in Table 1,was deposited on a plurality of the dielectric layer-formed sinteredniobium nitride bodies. Further, a carbon paste and then a silver pastewere laminated in this order on the dielectric layer-formed sinteredniobium nitride bodies. Then the thus-laminated product was encapsulatedwith an epoxy resin to give a capacitor.

The capacitance at 100 kHz and the LC value at 4 V were measured. Theresults are shown in Table 2.

TABLE 1 Example Other electrode and Electrode forming No. electricalconductivity(S · cm⁻¹) method Example 1 Chloranil complex of Repeat ofimmersion in tetrathiotetracene solution of the compound 2 × 10⁰described in the left column, and drying Example 2 Isoquinoline complexof Repeat of immersion in tetracyanoquinodimethane solution of thecompound 3 × 10⁰ described in the left column, and drying Example 3 Dopeof polyaniline in Repeat of oxidation toluenesulfonic acid reaction inaniline solution 3 × 10¹ Example 4 Dope of polypyrrole in Repeat ofoxidation toluenesulfonic acid reaction in pyrrole solution 5 × 10¹Example 5 Dope of polythiophene in Repeat of oxidation toluenesulfonicacid reaction in thiophene 4 × 10¹ solution Example 6 Mixture of leaddioxide and Repeat of oxidation lead sulfate reaction of lead acetate(lead dioxide 97 wt %) solution 5 × 10¹ Example 7 Mixture of manganeseThermal decomposition of dioxide and lead dioxide manganese nitrate(250° C. (lead dioxide 95 wt %) twice), then repeat of 5 × 10¹ oxidationreaction of lead acetate solution

EXAMPLES 8 AND 9

Powdery niobium nitride having an average particle diameter of 40 to 80μm and a bound nitrogen content of about 10,000 ppm by weight wassintered at 1,600° C. in vacuo to give sintered niobium nitride bodieshaving a diameter of 10 mm and a thickness of 1 mm, and containing poreshaving an average diameter of 7 μm with a porosity of 55%. The sinteredniobium nitride bodies were immersed in a bath of pentaethyl niobateliquid, and thereafter, the sintered niobium nitride bodies taken outfrom the bath were maintained at 85° C. in a steam and then dried at350° C. whereby a dielectric layer composed of niobium oxide was formedon the sintered niobium nitride bodies.

Each of chloranil complex of tetrathiotetracene (Example 8) and amixture of lead acetate and lead sulfate (Example 9) for forming anelectrode other than the electrode composed of the sintered niobiumnitride bodies was deposited on a plurality of the dielectriclayer-formed sintered niobium nitride bodies by the same proceduresemployed in Example 1 and Example 6, respectively. Further, a carbonpaste and then a silver paste were laminated in this order on thedielectric layer-formed sintered niobium nitride bodies. Then thelaminated product was encapsulated with an epoxy resin to give acapacitor. The properties of the capacitor were evaluated. The resultsare shown in Table 2.

COMPARATIVE EXAMPLES 1 AND 2

Powdery tantalum having an average particle diameter of 10 to 40 μm wassintered at 1,500° C. in vacuo to give sintered tantalum bodies having adiameter of 10 mm and a thickness of about 1 mm, and containing poreshaving an average diameter of 3 μm with a porosity of 45%. The sinteredtantalum bodies were treated in an aqueous phosphoric acid solution at avoltage of 20 V to form a tantalum oxide dielectric layer on the surfaceof each sintered body.

Each of chloranil complex of tetrathiotetracene (Comparative Example 1)and a mixture of lead acetate and lead sulfate (Comparative Example 2)for forming an electrode other than the electrode composed of thesintered tantalum bodies was deposited on a plurality of the dielectriclayer-formed sintered tantalum bodies by the same procedures employed inExample 1 and Example 6, respectively. Further, a carbon paste and thena silver paste were laminated in this order on the dielectriclayer-formed sintered tantalum bodies, and then, the thus-laminatedproduct was encapsulated with an epoxy resin by the same procedures asemployed in the above-mentioned Examples to give a capacitor. Theproperties of the capacitor were evaluated. The results are shown inTable 2.

COMPARATIVE EXAMPLES 3 AND 4

The procedures employed in Example 1 and Example 6 were repeated whereinthe powdery niobium was not nitrified and was sintered to give sinteredniobium bodies, and capacitors were made from the sintered niobiumbodies. The properties of the capacitors were evaluated. The results areshown in Table 2.

TABLE 2 Capacitance (100 kHz) LC (4 V) μF μA Example 1 55 0.9 Example 250 0.8 Example 3 60 1.2 Example 4 60 1.0 Example 5 55 1.2 Example 6 620.8 Example 7 60 1.0 Example 8 40 0.3 Example 9 40 0.3 ComparativeExample 1 24 0.02 Comparative Example 2 26 0.04 Comparative Example 3 5414 Comparative Example 4 57 18

EXAMPLE 10

The same sintered niobium nitride bodies as prepared in Example 1 wereimmersed in a bath of pentaethyl tantalate liquid, and thereafter, thesintered niobium nitride bodies taken out from the bath were maintainedat 85° C. in a steam and then dried at 450° C. whereby a dielectriclayer composed of tantalum oxide was formed on the sintered niobiumnitride bodies.

Then an electrolyte composed of a 5% solution ofisobutyltripropylammonium borontetrafluoride electrolyte in a mixedliquid of dimethylformamide and ethylene glycol, was applied onto thesintered niobium nitride bodies. The electrolyte-applied sinteredniobium nitride bodies were charged in a can, and the can was sealed togive a capacitor.

The properties of the capacitor were evaluated. The results are shown inTable 3.

COMPARATIVE EXAMPLE 5

The procedures employed in Example 10 were repeated to make a capacitorwherein sintered niobium bodies were used instead of the sinteredniobium nitride bodies with all other conditions remaining the same. Theproperties of the capacitor were evaluated. The results are shown inTable 3.

EXAMPLE 11

By the same procedures as employed in Example 1, sintered niobiumnitride bodies were made and then niobium oxide dielectric layers wereformed on the sintered niobium nitride bodies. An electrolyte wasapplied to the dielectric layer-formed sintered niobium nitride bodies,and the electrolyte-applied product was charged in a can and the can wassealed to give a capacitor by the same procedures as described inExample 10. The properties of the capacitor were evaluated. The resultsare shown in Table 3.

COMPARATIVE EXAMPLE 6

The procedures as employed in Example 11 were repeated to make acapacitor wherein sintered niobium bodies were used instead of thesintered niobium nitride bodies with all other conditions remaining thesame. The properties of the capacitor were evaluated. The results areshown in Table 3.

TABLE 3 LC (4 V) μA Example 10 0.3 Example 11 0.4 Comparative Example 59 Comparative Example 6 10

EXAMPLE 12

By the same procedures as employed in Example 1, sintered niobiumnitride bodies were made and then niobium oxide dielectric layers wereformed on the sintered niobium nitride bodies. The dielectriclayer-formed sintered niobium nitride bodies were immersed in an aqueousequimolar solution containing 0.01 mole/l of iron(II) sulfate andiron(III) sulfate, and then, an excessive amount of an aqueous sodiumhydroxide solution was added whereby triiron tetraoxide as the electrodeother than the niobium nitride electrode was formed on the dielectriclayer-formed sintered niobium nitride bodies. A carbon paste and then asilver paste were laminated in this order on the dielectric layer-formedsintered niobium nitride bodies, and then, the thus-laminated productwas encapsulated with an epoxy resin by the same procedures as employedin the above-mentioned Examples to give a capacitor. The triirontetraoxide used had an electrical conductivity of 10⁻³ S·cm⁻¹. Theproperties of the capacitor were evaluated. The results are shown inTable 4.

COMPARATIVE EXAMPLE 7

The procedures as employed in Example 12 were repeated to make acapacitor wherein sintered niobium bodies were used instead of thesintered niobium nitride bodies with all other conditions remaining thesame. The properties of the capacitor were evaluated. The results areshown in Table 4.

TABLE 4 Capacitance (100 kHz) LC (4 V) μF μA Example 12 38 0.7Comparative Example 7 38 16

INDUSTRIAL APPLICABILITY

The capacitor of the invention with an electrode composed of sinteredniobium nitride bodies exhibits excellent environmental stability andleak current (LC) characteristics.

Especially the capacitor having an electrode composed of sinteredniobium nitride bodies and the other electrode composed of at least oneingredient selected from organic semiconductors and inorganicsemiconductors, and having a niobium oxide dielectric interveningbetween the two electrodes has an enhanced capacitance per unit weightat a high frequency as well as excellent leak current (LC)characteristics. Therefore, the capacitor of the invention is suitablefor a smoothing circuit of a power source.

1. A capacitor comprising an electrode made of a sintered body made bysintering powdery niobium nitride which is made by nitrifying thesurfaces of particles of powdery niobium in a nitrogen gas atmosphere.2. The capacitor according to claim 1, wherein the amount of nitrogenbound to the niobium is in the range of from 10 to 200,000 ppm byweight.
 3. The capacitor according to claim 1, wherein the amount ofnitrogen bound to the niobium is in the range of from 100 to 50,000 ppmby weight.